Optical pickup for detecting thickness variation of a recording medium, and/or compensating for spherical aberration caused by thickness variation of a recording medium

ABSTRACT

An optical pickup for a recording medium includes a light beam division and detection unit including receiving portions dividing an incident light beam reflected on the recording medium into a first light beam portion and a second light beam portion around the first light beam portion and converting the first and second light beam portions into first and second detection signals, respectively. A thickness variation detection circuit detects a variation in thickness of the recording medium according to the first and second detection signals and outputs a thickness variation signal indicative thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Application No.2000-84211, filed Dec. 28, 2000, in the Korean Industrial PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup apparatus, and moreparticularly, to an optical pickup for detecting thickness variation ofa recording medium, and/or for compensating for spherical aberrationcaused by the thickness variation of a recording medium.

2. Description of the Related Art

In general, information recording/reproduction density increases as asize of a light spot focused on a recording medium in an optical pickupapparatus becomes smaller. The shorter a wavelength (λ) of light usedand the larger a numerical aperture (NA) of an objective lens, thesmaller the size of a light spot, which is expressed by equation (1):size of light spot α λ/NA  (1)

To reduce the size of the light spot focused on the recording medium inorder to obtain a higher recording density, there is a need to constructan optical pickup with a short wavelength light source, such as a bluesemiconductor laser, and an objective lens having a larger NA. A formatfor increasing recording capacity up to 22.5 GB with a 0.85-NA objectivelens, and for reducing the thickness of a recording medium to 0.1 mm isdesired so as to prevent degradation of performance caused by tilting ofthe recording medium. Here, the thickness of the recording medium isdefined as a distance from a light incident surface of the recordingmedium to an information recording surface.

As shown in equation (2) below, an spherical aberration W isproportional to a fourth power of the NA of the objective lens and to adeviation of the thickness of the recording medium. For this reason, ifan objective lens with a high NA of about 0.85 is adopted, the recordingmedium must have a uniform-thickness with a deviation less than ±3 μm.However, it is very difficult to manufacture the recording medium withinthe above thickness deviation range.

$\begin{matrix}{W_{\lbrack 40\rbrack} = {\frac{n^{2} - 1}{8n^{3}}( {N\; A} )^{4}\Delta\; d}} & (2)\end{matrix}$

FIG. 1 is a graph showing a relation between thickness deviation of therecording medium and wavefront aberration (optical path difference(OPD)) caused by a thickness deviation when a 400-nm light source and anobjective lens having an NA of 0.85 are used. As shown in FIG. 1, thewavefront aberration increases proportionally with the thicknessdeviation. Thus, when the objective lens having a high NA, for example,an NA of 0.85, is adopted, there is a need to correct for sphericalaberration caused by the thickness deviation of the recording medium.

FIG. 2 shows a conventional optical pickup capable of detectingvariation of the thickness of an optical disc 1, which is disclosed inJapanese Patent Laid-open Publication No. hei 12-57616. Referring toFIG. 2, the conventional optical pickup includes a light source 10emitting a light beam, a polarization beam splitter 11 transmitting orreflecting the light beam from the light source 10 incident on theoptical disc 1 according to the polarization of the light beam, and aquarter-wave plate 15 changing the polarization of an incident lightbeam. An objective lens 17 focuses the incident light beam to form alight spot on a recording surface 1 a of the optical disc 1. Acylindrical astigmatism lens 21 has an astigmatism affecting the lightbeam passed back through the objective lens 17, the quarter-wave plate15, and the polarization beam splitter 11 after being reflected from therecording surface 1 a of the optical disc 1. A photodetector 25 receivesthe light beam from the astigmatism lens 21. The conventional opticalpickup further includes a collimating lens 13 disposed between thepolarization beam splitter 11 and the quarter-wave plate 15, collimatingan incident diverging light beam from the light source 10 transmitted orreflected by the polarization beam splitter 11. A condensing lens 19 isdisposed between the polarization beam splitter 11 and the astigmatismlens 21. The polarization beam splitter 11, the collimator lens 13, thequarter-wave plate 15, the objective lens 17, the condensing lens 19,and the cylindrical astigmatism lens 21 are coaxially arranged.

Because the conventional optical pickup has the astigmatism lens 21which causes astigmatism to enable focus error signal detection, theintensity distribution of the light passed through the astigmatism lens21 after being reflected on the recording surface 1 a of the opticaldisc 1 varies according to the thickness t′ of the optical disc 1, asshown in FIGS. 3A through 3E. FIGS. 3A through 3E illustrate anintensity distribution of light passed through the astigmatism lens 21towards the photodetector 25, when the optical disc 1 adopted has athickness of 0.70 mm, 0.65 mm, 0.60 mm, 0.55 mm, and 0.50 mm,respectively, and the optical pickup of FIG. 2 is designed for a 0.6-mmthick optical disc.

Referring to FIG. 3C, when the optical disc 1 has a thickness of 0.60mm, which is a level of reference with respect to the other thicknesslevels (hereinafter, referred to as the reference thickness), theintensity distribution of the reflected light beam entering thephotodetector 25 is circular due to lack of spherical aberration, and issymmetrical around a center point. When the thickness of the opticaldisc 1 deviates from the reference thickness of 0.60 mm, sphericalaberration occurs as a result of the thickness deviation, and theintensity distribution of the reflected light beam passed through theastigmatism lens 21 and received by the photodetector 25 is asymmetricalabout the center point, as illustrated in FIGS. 3A, 3B, 3D, and 3E.

The photodetector 25 detects a variation in thickness of the opticaldisc 1 from a variation of intensity distribution of the received light.To this end, as shown in FIG. 4, the photodetector 25 of FIG. 2 includesfirst through fourth inner sections A1, B1, C1, and D1, and firstthrough fourth outer sections A2, B2, C2, and D2 surrounding the firstthrough fourth inner sections A1, B1, C1, and D1.

In a conventional optical pickup having the configuration describedabove, a thickness variation signal for the optical disc 1 is detectedby subtracting a sum of detection signals a2 and c2 of the first andthird outer sections A2 and C2 in one diagonal direction of thephotodetector 25, and the detection signals b1 and d1 of the second andfourth inner sections B1 and D1, respectively, in the other diagonaldirection, from a sum of detection signals a1 and c1 of the first andthird inner sections A1 and C1, respectively, in the one diagonaldirection, and detection signals b2 and d2 of the second and fourthouter sections B2 and D2, respectively, in the other diagonal direction.In other words, a thickness variation signal St′ for the optical disc 1can be detected from the detection signals a1, b1, c1, and d1 of thefirst through fourth inner sections A1, B1, C1, and D1, respectively, ofthe photodetector 25, and the detection signals a2, b2, c2, and d2 ofthe first through fourth outer sections A2, B2, C2 and D2, respectively,by using the following equation:St′=(a 1+c 1+b 2+d 2)−(a 2+c 2+b 1+d 1)  (3)

However, this mechanism of detecting variation of the thickness of theoptical disc can be applied to only optical pickups adopting theastigmatism lens. In other words, if an optical pickup does not includethe astigmatism lens, a thickness variation of an optical disc used inthe optical pickup cannot be detected.

SUMMARY OF THE INVENTION

Various objects and advantages of the invention will be set forth inpart in the description that follows and, in part, will be obvious fromthe description, or may be learned by practice of the invention.

To solve the above problem, it is an object of the present invention toprovide an optical pickup detecting variation of the thickness of arecording medium, and/or compensating for spherical aberration caused bythe thickness variation of a recording medium, without including anastigmatism lens to cause astigmatism at the light receiving side.

According to an object of the present invention, there is provided anoptical pickup including: a light source generating and emitting a lightbeam; an objective lens focusing the light beam from the light source toform a light spot incident on the recording medium; an optical pathchanger disposed on an optical path between the light source and theobjective lens, altering a traveling path of the incident light beam; alight beam division and detection unit dividing the incident light beampassed through the objective lens and the optical path changer into afirst light beam portion and a second light beam portion around thefirst light beam portion, and detecting first and second detectionsignals from the first and second light beam portions; and a thicknessvariation detection circuit detecting a variation in thickness of therecording medium by subtracting the second detection signal from thefirst detection signal and outputting a thickness variation signalindicative thereof.

The light beam division and detection unit may include a photodetectorincluding first and second light receiving portions dividing theincident light beam into the first light beam portion and the secondlight beam portion around the first light beam portion andphotoelectrically converting the first and second light beam portionsinto the first and second detection signals, respectively.

Preferably, the light beam division and detection unit includes: a lightbeam splitter including a first section and a second section, dividingthe incident light beam into the first light beam portion and the secondlight beam portion around the first light beam portion; and first andsecond photodetectors receiving the first and second light beam portionsfrom the light beam splitter, and photoelectrically converting the firstand second light beam portions into the first and second detectionsignals, respectively.

In another embodiment, the present invention provides for an opticalpickup including: a light source generating and emitting a light beam;an objective lens focusing an incident light beam from the light sourceto form a light spot on the recording medium; an optical path changerdisposed on an optical path between the light source and the objectivelens, altering a traveling path of the incident light beam; a light beamdivision and detection unit dividing the incident light beam passedthrough the objective lens and the optical path changer into a firstlight beam portion and second and third light beam portions around thefirst light beam portion, and detecting first, second, and thirddetection signals from the first, second and third light beam portions,respectively; a thickness variation detection circuit detecting avariation in thickness of the recording medium by subtracting a sum ofthe second and third detection signals from the first detection signaland outputting a thickness variation signal indicative thereof.

The light beam division and detection unit includes a photodetectorincluding first, second and third light receiving portions dividing theincident light beam into the first, second and third light beamportions, receiving the first, second and third light beam portions, andseparately and photoelectrically converting the first, second and thirdlight beam portions, respectively.

The light beam division and detection unit includes: a light beamsplitter including first, second, and third sections dividing theincident light beam into the first light beam portion and the second andthird light beam portions around the first light beam portion; a firstphotodetector receiving and photoelectrically converting the first lightbeam portion into the first detection signal; a second photodetectorreceiving and photoelectrically converting the second light beam portioninto the second detection signal; and a third photodetector receivingand photoelectrically converting the third light beam portion into thesecond detection signal.

The optical pickup according to the present invention may furtherinclude a spherical aberration compensation element on the optical pathbetween the optical path changer and the objective lens, drivenaccording to the thickness variation signal from the thickness variationdetection circuit to compensate for spherical aberration caused by thethickness variation of the recording medium.

The optical pickup according to the present invention may furtherinclude: a collimating lens on the optical path between the light sourceand the optical path changer, collimating a diverging light beam fromthe light source; and an actuator actuating the collimating lensaccording to the thickness variation signal detected by the thicknessvariation detection circuit, compensating for spherical aberrationcaused by the thickness variation of the recording medium.

It is another object of the present invention to provide for an opticalpickup for a recording medium, including: a light beam division anddetection unit including receiving portions dividing an incident lightbeam reflected on the recording medium into a first light beam portionand a second light beam portion around the first light beam portion andconverting the first and second light beam portions into first andsecond detection signals, respectively; and a thickness variationdetection circuit detecting a variation in thickness of the recordingmedium according to the first and second detection signals andoutputting a thickness variation signal indicative thereof.

These together with other objects and advantages, which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part thereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and advantages of the present invention will becomemore apparent by describing in detail preferred embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a graph showing the relation between thickness deviation of arecording medium and wavefront aberration (optical path difference(OPD)) caused by the thickness deviation;

FIG. 2 illustrates a conventional optical pickup detecting thicknessvariation of an optical disc;

FIG. 3A illustrates a distribution of light entering a photodetector ofthe optical pickup of FIG. 2, which is designed for 0.60-mm thickoptical discs and the optical disc has a thickness of 0.70 mm;

FIG. 3B illustrates a distribution of light entering the photodetectorof the optical pickup of FIG. 2, which is designed for 0.60-mm thickoptical discs and the optical disc has a thickness of 0.65 mm;

FIG. 3C illustrates a distribution of light entering the photodetectorof the optical pickup of FIG. 2, which is designed for 0.60-mm thickoptical discs and the optical disc has a thickness of 0.60 mm;

FIG. 3D illustrates a distribution of light entering the photodetectorof the optical pickup of FIG. 2, which is designed for 0.60-mm thickoptical discs and the optical disc has a thickness of 0.55 mm;

FIG. 3E illustrates a distribution of light entering the photodetectorof the optical pickup of FIG. 2, which is designed for 0.60-mm thickoptical discs and the optical disc has a thickness of 0.50 mm;

FIG. 4 is a plan view illustrating a configuration of the photodetectorshown in FIG. 2;

FIG. 5 shows an exemplary embodiment of an optical pickup in accordancewith the present invention;

FIG. 6A illustrates an intensity distribution of a light beam passedback through an objective lens and an optical path changer of theoptical pickup of FIG. 5 after being reflected from the recordingmedium, which is 10 μm thinner than a reference thickness for which theoptical pickup is designed;

FIG. 6B illustrates an intensity distribution of the light beam passedback through the objective lens and the optical path changer of theoptical pickup of FIG. 5 after being reflected from the recording mediumhaving a reference thickness of 0.1 mm;

FIG. 6C illustrates an intensity distribution of the light beam passedback through the objective lens and the optical path changer of theoptical pickup of FIG. 5 after being reflected from the recordingmedium, which is 10 μm thicker than the reference thickness for whichthe optical pickup is designed;

FIG. 7A illustrates a phase distribution of the light beam of FIG. 6A;

FIG. 7B illustrates a phase distribution of the light beam of FIG. 6B;

FIG. 7C illustrates a phase distribution of the light beam of FIG. 6C;

FIG. 8 illustrates exemplary embodiment of a photodetector of FIG. 5 anda thickness variation detection circuit;

FIG. 9 illustrates an alternative embodiment of the thickness variationdetection circuit of FIG. 8;

FIG. 10 illustrates alternative embodiment of the photodetector and thethickness variation detection circuit of FIG. 5;

FIG. 11 is a graph of a thickness variation signal and a sum of firstand second detection signals of first and second light receivingportions of the photodetector when the photodetector of the opticalpickup, according to the present invention, has the embodiment of FIG.8;

FIG. 12 is a graph of a thickness variation signal for the recordingmedium and a sum of the first, second, and third detection signals offirst, second and third light receiving portions of the photodetectorwhen the photodetector of the optical pickup, according to the presentinvention, has the embodiment of FIG. 10;

FIG. 13 illustrates an alternative embodiment of the optical pickupaccording to the present invention;

FIG. 14 illustrates an alternative embodiment of the optical pickupaccording to the present invention;

FIG. 15 is a plan view showing an embodiment of a light beam splitter ofFIG. 14;

FIG. 16 illustrates an alternative embodiment of the optical pickupaccording to the present invention; and

FIG. 17 is a plan view showing an embodiment of the light beam splitterof FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of an optical pickup according to the presentinvention is illustrated in FIG. 5. The optical pickup includes a lightsource 51 generating and emitting a light beam, and an objective lens 57focusing an incident light beam LB from the light source 51 to form alight spot on an information recording surface 50 a of a recordingmedium 50. An optical path changer is disposed on the optical pathbetween the light source 51 and the objective lens 57, for altering thetraveling path of the incident light beam LB. A photodetector 65 isprovided to divide and detect the light beam passed back through theobjective lens 57 and the optical path changer after being reflectedfrom the recording medium 50, and a thickness variation detectioncircuit 70 is provided to detect variation of the thickness of therecording medium 50 from a plurality of detection signals output fromthe photodetector 65. Here, the thickness t of the recording medium 50is defined as a distance between a light incident surface 50 b of therecording medium 50 and the information recording surface 50 a.Thickness variation refers to both thickness deviation according toposition on one recording medium and a difference in thickness betweendifferent optical discs.

The light source 51 may be a semiconductor laser, such as an edgeemitting laser or a vertical cavity surface emitting laser (VCSEL). Asthe optical path changer, a beam splitter 55 for transmitting andreflecting an incident light beam LB by a predetermined ratio may beadopted. Alternatively, the optical path changer may include both, apolarization beam splitter (not shown) for selectively transmitting orreflecting an incident light beam LB according to a polarization of theincident light beam LB, and a quarter-wave plate (not shown) disposed onthe optical path between the polarization beam splitter and theobjective lens 57, for changing the phase of an incident light beam LB.

In the optical pickup of FIG. 5 for recording and reproduction with anext generation digital versatile disc (DVD), so-called “high-definition(HD)-DVD”family recording medium, a blue-light semiconductor laser maybe adopted as the light source 51 to emit a light beam having awavelength of about 400–420 nm, for instance, a wavelength of about 405nm, and a lens having a numerical aperture (NA) of 0.7 or more, such as,an NA of 0.85, may be adopted as the objective lens 57.

The optical pickup according to the present invention may furtherinclude a collimating lens 53 on the optical path between the lightsource 51 and the beam splitter 55, for collimating a diverging lightbeam emitted from the light source 51. The optical pickup furtherincludes a sensing lens 59 on the optical path between the beam splitter55 and the photodetector 65, for condensing an incident light beam LB.The distance between the sensing lens 59 and the photodetector 65 isdetermined such that the light spot received by the photodetector 65 hasan appropriate size, for example, a diameter of about 100 μm.

The photodetector 65 as a light beam division and detection unit, isconstructed such that the photodetector 65 divides and detects the lightbeam passed back through the objective lens 57 and the optical pathchanger after being reflected on the recording medium 50, taking intoaccount a variation in the intensity distribution of the light beamaccording to thickness variation of the recording medium 50.

For illustrative purposes, the objective lens 57 has an NA of 0.85, theoptical pickup is designed for the recording medium 50 having athickness of 0.1 mm, and the light source 51 emits a 400-nm light beam.In this case, FIGS. 6A through 6C and FIGS. 7A through 7C illustrateintensity distribution and phase distribution of the light beam passedback through the objective lens 57 and the optical path changer afterbeing reflected from the recording medium 50, with respect to thevariation in the thickness of the recording medium 50.

In particular, FIG. 6A illustrates the intensity distribution of thelight passed through the optical path changer after being reflected onthe recording medium 50 which is 10 μm thinner than the 0.1 mm thicknessfor which the optical pickup is designed (hereinafter referred to as thereference thickness). FIG. 7A illustrates the phase distribution of thelight beam of FIG. 6A. FIG. 6B illustrates the intensity distribution ofthe light beam for the recording medium 50 having a thickness equal tothe reference thickness of 0.1 mm. FIG. 7B shows the phase distributionof the light beam of FIG. 6B. FIG. 6C illustrates the intensitydistribution of the light beam for the recording medium 50, which is 10μm thicker than the reference thickness. FIG. 7C illustrates the phasedistribution of the light beam of FIG. 6C.

Referring to FIGS. 6A and 7A, when a portion of the recording medium 50,onto which the light beam is focused, is thinner than the referencethickness, the intensity distribution of the light beam is weaker at acentral axis, and increases with increased distance from the centralaxis. Also, the phase distribution of the light beam appears like twinpeaks, that is, symmetrical with respect to the central axis. Referringto FIGS. 6B and 7B, when a portion of the recording medium 50 onto whichthe light beam is focused is equal to the reference thickness, theintensity distribution of the light beam is uniform and the phasedistribution is uniform. Referring to FIGS. 6C and 7C, when a portion ofthe recording medium 50 onto which the light beam is focused is thickerthan the reference thickness, the intensity distribution and the phasedistribution of the light beam are inverted with respect to those ofFIGS. 6A and 7A.

As illustrated in FIGS. 6A through 6C and FIGS. 7A through 7C, theintensity distribution and the phase distribution of the light beamaccording to the thickness variations of the recording medium 50 varysymmetrically around the central axis and are opposite with respect toan inverse in thickness variations. Furthermore, variations in theintensity distribution and phase spectrum of the light beam caused by anincrease in the thickness of the recording medium above the referencethickness are opposite to variation in the intensity distribution andthe phase spectrum of the light beam caused by a decrease in thethickness of the recording medium below the reference thickness.

For this reason, the photodetector 65 may be constructed such that thephotodetector 65 separately divides the incident light beam LB into afirst light portion corresponding to the optical axis and a second lightportion around the periphery of the optical axis. For example, as shownin FIG. 8, the photodetector 65 may include first and second lightreceiving portions A and B, respectively, for dividing the incidentlight beam LB into a first light beam portion corresponding to thecentral optical axis, and a second light beam portion around the firstlight beam portion, and for photoelectrically converting the first andsecond light beam portions into first and second detection signals a andb, respectively. In this case, the first light receiving portion A ofthe photodetector 65 may have a circular or rectangular form to allowdivision of the incident light beam LB into the first light beamportion, corresponding to the central optical axis, and the second lightbeam portion around the first light beam portion, and to allow separatedetection of the two portions.

When the photodetector 65 includes the first and second light receivingportions A and B, as shown in FIG. 8, the thickness variation detectioncircuit 70 is constructed with a subtractor 71 for subtracting thesecond detection signal b corresponding to the second light beam portionof the second light receiving portion B from the first detection signala corresponding to the first light beam portion of the first lightreceiving portion A and outputting a result of the subtraction as athickness variation signal St to the recording medium 50. In this case,as shown in FIG. 9, the thickness variation detection circuit 70 mayfurther include a gain controller 73 for amplifying at least one of thefirst and second detection signals a and b by a predetermined gainfactor k prior to the subtraction by the subtractor 71, such that anoffset of the thickness variation signal St can be adjusted.

Alternatively, as shown in FIG. 10, the photodetector 65 may includefirst, second, and third light receiving portions D, E, and F,respectively, for dividing the incident light beam LB into a first lightbeam portion aligned with an optical axis, and second and third lightbeam portions around the first light beam portion, and forphotoelectrically converting the first, second, and third light beamportions into first, second, and third detection signals d, e, and f,respectively. The first, second, and third light receiving portions D,E, and F may be arranged in a direction corresponding to either atangential or radial direction of the recording medium 50.

When the photodetector 65 is constructed as illustrated in FIG. 10, thethickness variation detection circuit 70 detects the variation in thethickness of the recording medium 50 by subtracting the sum of thesecond and third detection signals e and f, of the second and thirdlight receiving portions E and F, from the first detection signal d ofthe first light receiving portion D. As illustrated in FIG. 9, thethickness variation detection circuit 70 may be constructed such thatthe thickness variation detection circuit 70 may amplify at least one ofthe first, second and third detection signals d, e, and f, respectively,by a predetermined gain factor k, and process the first, second, andthird detection signals d, e, and f, respectively, so that the offset ofthe thickness variation signal St may be adjusted.

For the photodetector 65 illustrated in FIGS. 8 and 10, a size of thefirst light receiving portion A of FIG. 8 and a size of the first lightreceiving portion D of FIG. 10 are determined such that the first lightreceiving portion A and the first light receiving portion D receive10–90% of the entire incident light beam LB.

Turning back to FIG. 5, the optical pickup according to the presentinvention may further include a spherical aberration compensationelement 75 on the optical path between the optical path changer and theobjective lens 57, which is driven according to the thickness variationsignal St produced by the thickness variation detection circuit 70,thereby compensating for spherical aberration caused by thicknessvariation of the recording medium 50.

The spherical aberration compensation element 75 may be a liquid crystalplate manufactured by injecting liquid crystals between two sheets oftransparent substrates having electrode patterns. Due to the anisotropicproperty of the liquid crystal with respect to a refractive index, thephase of the light beam passing through the liquid crystal platechanges. In particular, the liquid crystal plate is driven according tothe thickness variation signal St such that a shape of a wavefront ofthe light beam passing the liquid crystal plate is changed into aninverse shape of spherical aberration caused by the thickness variationof the recording medium 50, thereby compensating for the sphericalaberration caused by the thickness variation of the recording medium 50.In this case, a driving circuit for driving the spherical aberrationcompensation element 75 may be included in or separate from thethickness variation detection circuit 70.

FIG. 11 is a graph of the thickness variation signal St and a sum Ssumof the first and second detection signals a and b of the first andsecond light receiving portions A and B of the photodetector 65 versusthe thickness variation of the recording medium 50 when thephotodetector 65 of the optical pickup of the present invention has theembodiment of FIG. 8. FIG. 12 is a graph of the thickness variationsignal St and the sum Ssum of the first, second, and third detectionsignals d, e, and f of the first, second, and third light receivingportions D, E, and F of the photodetector 65 versus the thicknessvariation of the recording medium 50, when the photodetector 65 of theoptical pickup of the present invention has the embodiment of FIG. 10.As shown in FIGS. 11 and 12, the variation of the thickness variationsignal St detected by the thickness variation detection circuit 70 withrespect to the thickness variation of the recording medium 50 isrelatively larger than the variation of the sum Ssum of the detectionsignals detected by the photodetector 65.

As described with reference to FIGS. 11 and 12, the variation in thethickness of the recording medium 50 can be detected by the opticalpickup of the present invention. Thus, the spherical aberration causedby the thickness variation of the recording medium 50 can be correctedby driving the spherical aberration compensation element 75 according tothe thickness variation signal St.

Referring to FIG. 13, for the purpose of compensating for sphericalaberration caused by the thickness variation of the recording medium 50,the optical pickup according to the present invention may include anactuator 80 actuating the collimating lens 53 along the optical axisaccording to the thickness variation signal St produced by the thicknessvariation detection circuit 70 instead of the spherical aberrationcorrection element 75 of FIG. 5.

FIG. 14 illustrates an alternative embodiment of the optical pickupaccording to the present invention. In the present embodiment, ratherthan the photodetector 65 having the embodiment shown in FIG. 8, a lightbeam splitter 160, and first and second photodetectors 165 a and 165 bare used as a light beam division and detection unit. In FIG. 14, thesame elements as in FIG. 5 are denoted by the same reference numerals,and descriptions thereof will not be provided here.

The light beam splitter 160 includes first and second sections A′ and B′as shown in FIG. 15 for dividing the incident light beam LB into a firstlight beam portion on the optical axis, and a second light beam portionaround the first light beam portion. The first section A′ directlytransmits, for example, the first light beam portion of the incidentlight beam LB, or diffracts the first light beam portion of the incidentlight beam LB into a 0th-order beam, so that the transmitted ordiffracted light beam is received by the first photodetector 165 a. Thesecond section B′ diffracts, for example, the second light beam portionof the incident light beam LB so that a +1^(st)-order or −1^(st)-orderbeam is received by the second photodetector 165 b. The light beamsplitter 160 may be a hologram optical element (HOE), which has in thefirst section A′ a through hole, a direct transmit portion, or ahologram pattern for diffracting the incident light beam LB andtransmitting a resulting 0^(th)-order light beam, and has in the secondsection B′ a hologram pattern for diffracting the incident light beam LBand transmitting a resulting +1^(st)-order or −1^(st)-order light beam.

In the optical pickup according to the embodiment of the presentinvention shown in FIG. 14, the principle of detecting the thicknessvariation signal St for the recording medium 50 from the first andsecond detection signals a and b of the first and second photodetectors165 a and 165 b, and correcting the spherical aberration caused by thethickness variation of the recording medium 50 by driving the sphericalaberration compensation element 75 according to the thickness variationsignal St, is the same as in the previous embodiments. Alternatively,the optical pickup of FIG. 14 may include the actuator 80 for actuatingthe collimating lens 53 along the optical axis, as shown in FIG. 13,thereby compensating for spherical aberration caused by thicknessvariation of the recording medium 50.

FIG. 16 illustrates an alternative embodiment of the optical pickupaccording to the present invention. In the present embodiment, insteadof the photodetector 65 having the divided configuration shown in FIG.10, a light beam splitter 260, and first, second and thirdphotodetectors 265 d, 265 e, and 265 f are used as a light beam divisionand detection units.

In FIG. 16, the same elements as in FIG. 5 are denoted by the samereference numerals, and descriptions thereof will not be provided here.

In the embodiment shown in FIG. 16, the light beam splitter 260 includesfirst, second and third sections D′, E′ and F′, as shown in FIG. 17, fordividing the incident light beam LB into a first light beam portion onthe optical axis, and second and third light beam portions around thefirst light beam portion, based on a principle similar to the light beamsplitter 160 of FIG. 15. The first section D′ directly transmits, forexample, the first light beam portion of the incident light beam LB, ordiffracts the first light beam portion of the incident light beam LB andtransmits the resulting 0^(th)-order beam, so that the transmitted ordiffracted light beam is received by the first photodetector 265 d. Thesecond section E′ diffracts, for example, the second light beam portionof the incident light beam LB so that the +1^(st)-order or −1^(st)-orderdiffracted light beam is received by the second photodetector 265 e. Thethird section F′ diffracts, for example, the third light beam portion ofthe incident light beam LB so that the −1^(st)-order or +1^(st)-orderdiffracted light beam is received by the third photodetector 265 f. Thelight beam splitter 160 may be an HOE, which has in the first section D′a through hole, a direct transmit portion, or a hologram pattern fordiffracting the incident light beam LB, and has in both, the second andthird sections E′ and F′, a hologram pattern for diffracting theincident light beam LB.

In the optical pickup according to the embodiment of the presentinvention shown in FIG. 16, the principle of detecting the thicknessvariation signal St for the recording medium 50 from the first, second,and third detection signals d, e, and f of the first, second, and thirdphotodetectors 265 d, 265 e, and 265 f, and compensating for thespherical aberration caused by thickness variation of the recordingmedium 50 is the same as the principle described in the previousembodiments. In particular, the principle of detecting the thicknessvariation signal St for the recording medium 50 used by the embodimentof FIG. 16 includes driving the spherical aberration compensationelement 75 or the collimating lens 53 according to the detectedthickness variation signal St.

As described above, in the optical pickups according to the presentinvention, a single photodetector having a divided configuration or alight beam splitter and a plurality of photodetectors is used as a lightbeam division and detection unit. The light beam passed through anobjective lens and an optical path changer after having been reflectedfrom a recording medium is divided and detected by the light beamdivision and detection unit, taking into account a variation inintensity distribution of the light caused by thickness variation of therecording medium. A thickness variation signal is detected by processingthe detection signals from the photodetector. Thus, the variation inthickness of the recording medium can be detected with the opticalsystem without an astigmatism lens installed to cause astigmatism at thelight receiving side of the optical pickup. Spherical aberration causedby the thickness variation of the recording medium can be corrected bydriving a spherical aberration compensation element or a collimatinglens along the optical axis according to the detected thicknessvariation signal.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade thereto without departing from the spirit and scope of theinvention as defined by the appended claims.

1. An optical pickup for a recording medium, comprising: a light sourcegenerating and emitting a light beam; an objective lens focusing thelight beam from the light source to form a light spot on the recordingmedium; an optical path changer disposed on an optical path between thelight source and the objective lens, altering a traveling path of thelight beam incident on the recording medium; a light beam division anddetection unit dividing the incident light beam passed through theobjective lens and the optical path changer after being reflected fromthe recording medium into a first light beam portion and a second lightbeam portion encircling the first light beam portion, and detectingfirst and second detection signals from the first and second light beamportions; and a thickness variation detection circuit detecting avariation in thickness of the recording medium by subtracting the seconddetection signal from the first detection signal and outputting athickness variation signal indicative thereof.
 2. The optical pickup asrecited in claim 1, wherein the light beam division and detection unitcomprises: a photodetector comprising first and second light receivingportions dividing the incident light beam into the first light beamportion and the second light beam portion encircling the first lightbeam portion and photoelectrically converting the first and second lightbeam portions into the first and second detection signals, respectively.3. The optical pickup as recited in claim 1, wherein the light beamdivision and detection unit divides and detects the light beam as acircular or rectangular first light beam portion and as a circular orrectangular second light beam portion surrounding the first light beamportion.
 4. The optical pickup as recited in claim 3, wherein the firstlight beam portion corresponds to 10–90% of the incident light beam. 5.The optical pickup as recited in claim 1, further comprising: aspherical aberration compensation element on the optical path betweenthe optical path changer and the objective lens, driven according to thethickness variation signal from the thickness variation detectioncircuit to compensate for spherical aberration caused by the thicknessvariation of the recording medium.
 6. The optical pickup as recited inclaim 1, wherein the thickness of the recording medium is a distancebetween a light incident surface of the recording medium and aninformation recording surface of the recording medium.
 7. The opticalpickup as recited in claim 2, wherein the optical path changercomprises: a beam splitter transmitting the light beam from the lightsource to the recording medium through the objective lens and reflectingthe incident light beam through the objective lens to the photodetectorby a predetermined ratio.
 8. The optical pickup as recited in claim 1,further comprising: a collimating lens on the optical path between thelight source and the optical path changer collimating the light beam,which is diverging, from the light source; and a sensing lens on theoptical path between the optical path changer and the light beamdivision and detection unit condensing the incident light beam.
 9. Theoptical pickup as recited in claim 1, wherein the optical path changercomprises: a polarization beam splitter selectively transmitting thelight beam from the light source to the recording medium and reflectingthe incident light beam to the photodetector according to a polarizationof the incident light beam; and a quarter-wave plate disposed on theoptical path between the polarization beam splitter and the objectivelens changing a phase of the incident light beam.
 10. The optical pickupas recited in claim 1, further comprising: a blue-light semiconductorlaser emitting the light beam having a wavelength between 400 nm and 420nm, wherein the objective lens comprises a numerical aperture of atleast 0.7.
 11. The optical pickup as recited in claim 2, wherein a sizeof the light receiving portion is determined where the first lightreceiving portion receives 10–90% of the entire incident light beam. 12.An optical pickup for a recording medium, comprising: a light beamdivision and detection unit comprising receiving portions dividing anincident light beam reflected from the recording medium into a firstlight beam portion and a second light beam portion encircling the firstlight beam portion and converting the first and second light beamportions into first and second detection signals, respectively; and athickness variation detection circuit detecting a variation in thicknessof the recording medium according to the first and second detectionsignals and outputting a thickness variation signal indicative thereof.13. The optical pickup according to claim 12, wherein the thicknessvariation detection circuit detects the variation in thickness of therecording medium by subtracting the second detection signal from thefirst detection signal and outputs a thickness variation signalindicative thereof.
 14. The optical pickup according to claim 12,further comprising: a light source generating and emitting a light beam;an objective lens focusing the light beam from the light source to forma light spot incident on the recording medium; and an optical pathchanger disposed on an optical path between the light source and theobjective lens, altering a traveling path of the incident light beam.15. The optical pickup as recited in claim 12, further comprising: aspherical aberration compensation element on the optical path betweenthe optical path changer and the objective lens, driven according to thethickness variation signal from the thickness variation detectioncircuit to compensate for spherical aberration caused by the thicknessvariation of the recording medium.
 16. The optical pickup as recited inclaim 14, wherein the thickness of the recording medium is a distancebetween a light incident surface of the recording medium and aninformation recording surface of the recording medium.
 17. The opticalpickup as recited in claim 14, wherein the optical path changercomprises: a beam splitter transmitting the light beam from the lightsource to the recording medium through the objective lens and reflectingthe incident light beam through the objective lens to the photodetectorby a predetermined ratio.
 18. The optical pickup as recited in claim 14,further comprising: a collimating lens on the optical path between thelight source and the optical path changer collimating a diverging lightbeam from the light source; and a sensing lens on the optical pathbetween the optical path changer and the light beam division anddetection unit condensing the incident light beam.
 19. The opticalpickup as recited in claim 14, wherein the optical path changercomprises: a polarization beam splitter selectively transmitting thelight beam from the light source to the recording medium and reflectingthe incident light beam to the photodetector according to a polarizationof the incident light beam; and a quarter-wave plate disposed on theoptical path between the polarization beam splitter and the objectivelens changing a phase of the incident light beam.
 20. The optical pickupas recited in claim 14, wherein the light beam division and detectionunit comprises: a light beam splitter comprising a first section and asecond section dividing the incident light beam into the first lightbeam portion and the second light beam portion around the first lightbeam portion; and a photodetector receiving and photoelectricallyconverting the first and second light beam portions into the first andsecond detection signals, respectively.
 21. The optical pickup asrecited in claim 14, further comprising: a blue-light semiconductorlaser emitting the light beam having a wavelength between 400 nm and 420nm, wherein the objective lens comprises a numerical aperture of atleast 0.7.
 22. An optical pickup for a recording medium, comprising: alight source generating and emitting a light beam; an objective lensfocusing the light beam from the light source to form a light spot onthe recording medium; an optical path changer disposed on an opticalpath between the light source and the objective lens, altering atraveling path of the light beam incident on the recording medium; alight beam division and detection unit dividing the incident light beampassed through the objective lens and the optical path changer afterbeing reflected from the recording medium into a first light beamportion and a second light beam portion encircling the first light beamportion, and detecting first and second detection signals from the firstand second light beam portions; and a thickness variation detectioncircuit detecting a variation in thickness of the recording medium bysubtracting the second detection signal from the first detection signaland outputting a thickness variation signal indicative thereof tocompensate for spherical aberration, wherein the optical pickup isexclusive of an astigmatism lens causing astigmatism affecting the lightbeam passed back through the objective lens, and the optical pathchanger after being reflected from the recording surface of the opticaldisc.
 23. An optical pickup for a recording medium, comprising: a lightsource generating and emitting a light beam; an objective lens focusingthe light beam from the light source to form a light spot on therecording medium; an optical path changer disposed on an optical pathbetween the light source and the objective lens, altering a travelingpath of the light beam incident on the recording medium; a light beamdivision and detection unit dividing the incident light beam passedthrough the objective lens and the optical path changer after beingreflected from the recording medium into a first light beam portion anda second light beam portion around the first light beam portion, anddetecting first and second detection signals from the first and secondlight beam portions; a thickness variation detection circuit detecting avariation in thickness of the recording medium by subtracting the seconddetection signal from the first detection signal and outputting athickness variation signal indicative thereof; and a sphericalaberration compensation element on the optical path between the opticalpath changer and the objective lens, driven according to the thicknessvariation signal from the thickness variation detection circuit tocompensate for spherical aberration caused by the thickness variation ofthe recording medium.
 24. An optical pickup for a recording medium,comprising: a light beam division and detection unit comprisingreceiving portions dividing an incident light beam reflected from therecording medium into a first light beam portion and a second light beamportion around the first light beam portion and converting the first andsecond light beam portions into first and second detection signals,respectively; and a thickness variation detection circuit detecting avariation in thickness of the recording medium according to the firstand second detection signals and outputting a thickness variation signalindicative thereof; and a spherical aberration compensation element onthe optical path between the optical path changer and the objectivelens, driven according to the thickness variation signal from thethickness variation detection circuit to compensate for sphericalaberration caused by the thickness variation of the recording medium.